1. Field of the Invention
The present invention relates to a method and apparatus for fine frequency synchronization which are used in a wireless broadband (WiBro) system, and more particularly, to a method and apparatus for highly precisely estimating a fine frequency offset without using a Global Positioning System (GPS) receiver, in an environment where signals of other base stations or repeaters exist as interference signals in a frame signal which is received through a wireless link.
2. Description of the Related Art
In a next-generation mobile communication system, a subminiature base station and an intelligent wireless repeater have to be necessarily introduced in order to reduce costs required for adding base stations, expand the radius of a service cell, and increase a capacity that is to be assigned to congested regions. In order to allow an apparatus which is newly added to a system to transmit high quality data to a terminal, wireless repeater network synchronization and system clock synchronization become important technical components.
Particularly, in order to allow a subminiature base station and a wireless repeater to transmit high quality data to a terminal that is within a service region, in a wireless broadband (WiBro) system which is being developed as a next-generation mobile communication system, stable time synchronization and frequency synchronization have to be maintained. Here, in order to achieve precise time synchronization and frequency synchronization, it is preferable to receive and use signals of a GPS satellite including an atomic clock. However, when all of a plurality of wireless apparatuses time maintain time synchronization and frequency synchronization using GPS receivers, problems exist in that costs increase greatly and installation of the wireless apparatuses is limited because no GPS signal can be received in most indoor places.
In order to resolve the problems, there can be considered a method of acquiring time and frequency synchronization on the basis of a WiBro frame signal which is received through a wireless channel from a target base station (a base station which is in an optimum signal reception state among adjacent base stations), without using a GPS receiver. For time and frequency synchronization without using a GPS receiver, a general Orthogonal Frequency-Division Multiplexing (OFDM) synchronization process can be applied.
Paul H. Moose has proposed a method for high-speed frame signal detection and time and frequency synchronization based on a general OFDM transmission method, in “A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction,” IEEE Trans. Communications, vol. 42, No. 10, October 1994, pp 2908-2914. The Moose's method uses correlative values in a time domain. The technical concept of the Moose's method will be simply described below.
FIG. 1 is a view for explaining a time synchronization method based on the Moose's method.
Referring to FIG. 1, two training symbols (that is, first and second training symbols y(n) and y(n+1)) with the same complex value, each having a length of L, are transmitted as a preamble, and a receiving terminal sets a time window having a length of 2 L for synchronization, and then acquires time synchronization using a timing metric M(d) defined in Equation 1.
                                          M            ⁡                          (              d              )                                =                                                                                      P                  ⁡                                      (                    d                    )                                                                              2                                                      (                                  R                  ⁡                                      (                    d                    )                                                  )                            2                                      ,                            (        1        )            where d represents a time index for the first training symbol y(n).
Here, P(d) is a correlative value in a time domain, and represented as a sum of products between training symbols that are successively received, according to Equation 2.
                              P          ⁡                      (            d            )                          =                              ∑                          m              =              0                                      L              -              1                                ⁢                      y            *                          (                              d                +                m                            )                        ⁢                          y              ⁡                              (                                  d                  +                  m                  +                  L                                )                                                                        (        2        )            
R(d) is an energy value of the second training symbol y(n+1) which is received, and represented by Equation 3, below.
                              R          ⁡                      (            d            )                          =                              ∑                          m              =              0                                      L              -              1                                ⁢                                                                  y                ⁡                                  (                                      d                    +                    m                    +                    L                                    )                                                                    2                                              (        3        )            
Meanwhile, after time synchronization is acquired by Equation 1, a frequency offset is estimated, using optimal time synchronization and Equation 2 which represents the correlative value in the time domain. The frequency offset is represented by Equation 4.
                    ε        =                              1                          2              ⁢              π              ⁢                                                          ⁢              L                                ⁢          angle          ⁢                                          ⁢                      (                          P              ⁡                              (                d                )                                      )                                              (        4        )            
Now, the time synchronization method based on the Moose's method using the correlative value in the time domain, as described above, is applied to the WiBro system.
In the case of the WiBro system, in order to allow a receiving terminal to acquire time and frequency synchronization, a first OFDM symbol of a frame is transmitted as a preamble. The preamble consists of three types of preamble segments each having 284 subcarriers. As such, since the first OFDM symbol of the frame consists of three identical training symbols in a time domain, the general OFDM synchronization method (that is, the Moose's method) as described above can be applied to two training symbols that are adjacent to each other.
That is, time synchronization can be acquired by applying the timing metric M(d) defined in Equation 1 to two training symbols that are adjacent to each other. Also, by applying Equation 4 of estimating a frequency offset using the acquired optimal time synchronization and the correlative value (Equation 2) in the time domain, frequency synchronization can be acquired.
However, it is difficult to apply the frequency synchronization method based on the Moose's method to an actual WiBro environment in which a variety of interference signals exist. The reason is because a signal which is received through a wireless channel is a sum of a variety of interference signals that are received from other peripheral base stations or repeaters as well as from a target base station, and thus a frequency offset cannot be precisely estimated due to the influence of the interference signals. Particularly, in the case of a repeater which is installed in a hot-spot region of an inner city, the influence of such interference signals increases greatly.
Actual requirements for frequency synchronization of a WiBro wireless repeater are as follows. A requirement for reference frequency accuracy of a base station is about 2 PPM, and inter base station frequency synchronization accuracy which is required for hand-off is about 1% of an OFDM subcarrier interval. That is, inter base station frequency synchronization accuracy corresponding to an offset of about 97 Hz is required.
Accordingly, frequency synchronization accuracy for each base station, which is required to support hand-off, becomes about 48 Hz. As such, in order to allow a wireless repeater to relay signals of a base station and transmit high quality data to a terminal which is within a service region and particularly to support hand-off, precise frequency synchronization which stably satisfies the frequency synchronization requirement (that is, the frequency synchronization accuracy of about 48 Hz) is necessary.
However, when the Moose's method of estimating a frequency offset using correlative values in a time domain, as described above, is applied, a problem exists in that precise frequency offset estimation capable of satisfying frequency synchronization accuracy of about 48 Hz is very difficult, in a WiBro environment where a variety of interference signals exist in a wireless link.